1. Field of the Invention
The present invention relates to a semiconductor device having an electrostatic protection circuit for protecting the internal circuit from electrostatic breakdown.
2. Description of the Related Art
A semiconductor device such as an IC or LSI incorporates an electrostatic protection circuit to protect its internal circuit from an overcurrent such as an ESD (Electro Static Discharge) surge. To protect an internal circuit from an ESD surge input to an I/O terminal, for example, U.S. Pat. No. 6,765,772 proposes an ESD protection circuit using a GGNMOSFET (Grounded Gate N-channel MOSFET) and an SGPMOSFET (Source connected Gate P-channel MOSFET). The GGNMOSFET and SGPMOSFET have gates connected to a source and bulk (back gate) and are in an OFF state.
When an ESD surge is applied to the drain of the GGNMOSFET or SGPMOSFET, a BJT (Bipolar Junction Transistor) parasitic on the MOSFET is turned on to remove the ESD surge to the ground or power supply, though the MOSFET itself remains OFF. The parasitic BJT includes a drain serving as a collector, a source serving as an emitter, and a bulk serving as a base.
The parasitic BJT operates on the basis of the following principle. First, the ESD surge generates a high voltage in the junction on the drain side. This causes junction breakdown and flows a current to between the drain and the bulk. The base of the parasitic BJT is connected to the bulk via a well region having a relatively high resistance. Hence, the base is biased by an IR drop. The parasitic BJT is consequently turned on to remove the ESD surge.
When an I/O buffer has a GGNMOSFET and SGPMOSFET for ESD protection, their current paths are connected in parallel to those of a buffer NMOSFET and buffer PMOSFET included in the output buffer.
If an ESD surge is applied to the I/O terminal of this circuit arrangement, the gate potential of the MOSFET of the output buffer may rise to a level high enough to turn on the MOSFET. For example, assume that a positive ESD surge is applied to the I/O terminal. A buffer PMOSFET and SGPMOSFET are connected to the I/O terminal. A p-n junction diode is parasitic between the drain and bulk of the buffer PMOSFET or SGPMOSFET. For this reason, the power supply terminal connected to the bulk is stepped up to a positive potential. The logic circuit to control the I/O buffer is operable upon ESD surge application. The potential of the control signal of the logic circuit is indefinite. Hence, the gate potential may rise and turn on the buffer MOSFET upon surge application, depending on the arrangement of the logic circuit.
When the gate potential is given, and the ESD surge steps up the drain of the buffer NMOSFET, the parasitic BJT of the buffer NMOSFET is turned on at a drain voltage lower than that of the GGNMOSFET and SGPMOSFET (these MOSFETs have gate potentials that turn them off). This is because the inverted drain current of the MOSFET is superimposed on the emitter current of the parasitic BJT. As described above, if the buffer NMOSFET operates as a discharge device at a potential lower than that that makes the GGNMOSFET and SGPMOSFET operate as discharge devices, the current concentrates at the buffer NMOSFET, and no sufficient breakdown voltage can be ensured.
As a measure, U.S. Pat. No. 6,765,772 described above proposes a circuit that causes a diode string (voltage detecting device) to detect an increase in power supply voltage caused by an ESD surge and fixes the gate of the buffer NMOSFET to a low level through a switching circuit and driving circuit, thereby forcibly turning off the buffer NMOSFET. U.S. Patent Application Publication No. 2004/0105201A1 also describes a measure based on the same concept.
However, as the degree of integration increases, and the power supply voltage decreases, the breakdown voltage of each MOSFET included in the internal circuit becomes low. It is therefore difficult to sufficiently protect the internal circuit by the ESD protection circuit. A semiconductor device capable of effectively protecting the internal circuit from electrostatic breakdown is therefore in demand.